NiTi alloys display excellent shape memory behaviour and possess outstanding specific work output, along with other unusual properties such as superelastic behaviour, good damping behaviour, and good erosion and wear resistance. These properties are widely exploited, near room temperature, in commercially available NiTi alloys for connectors, couplings, valves, seals, coatings, actuators, and various medical and dental devices. Commercially available NiTi alloys are often marketed as “Nitinol” and have an approximately 48.5-51 at % Ni and 48.5-51 at % Ti content. The uses for a material with similar properties, especially high work output, but also capable of being used at elevated temperatures are nearly limitless. Designs incorporating high temperature shape memory alloys (“HTSMA”) are common and the patent literature is full of such examples. However, as with many new technologies, materials development has seriously lagged component design and no commercial shape memory alloys currently exist for high temperature applications.
The unique behavior of NiTi and other shape memory alloys is based on the reversible temperature-dependent austenite-to-martensite phase transformation. In practice, these alloys are easily deformable by twinning from an original shape while in their lower temperature form (martensite) and upon heating through the transformation to austenite, will revert back to their original shape (the shape prior to deformation of the martensite phase). This behavior is the basis for such properties as shape memory effect and superelasticity. In addition, a subset of the alloys that display shape memory effect can recover their original shape even when a substantial bias force is applied to the material. In this way, the alloy is capable of performing work, acting as a solid state actuator. NiTi, in particular, is capable specific work output with equivalent or higher energy density than pneumatic actuators or D.C. motors.
The reversible transformation of Ni—Ti shape memory alloys between the austenite and martensite phases occurs over two different temperature ranges, which are characteristic of the specific alloy. When these alloys are heated they will change from their martensite form to austenite. The temperature at which this transformation starts is the austenite start temperature (As) and the temperature at which this transformation from martensite to austenite is complete is called the austenite finish temperature (Af). When the alloy is cooled from high temperature the transformation is reversed and the austenite will begin transforming to martensite at the martensite start temperature (Ms) and will be completely converted to martensite at the martensite finish temperature (Mf). Therefore, these transformation temperatures determine over what temperature range the shape memory and other effects can be observed. For NiTi, the maximum Ms temperature is observed in stoichiometric Ni-50 at. % Ti and Ti-rich NiTi alloys and is about 60° C. (Otsuka and Ren, Prog. Mater. Sci. 50 (2005) 511). The practical use temperature for Ni—Ti alloys is even lower, because the martensite finish temperature or reset temperature for these alloys is 10-20° C. below the Ms temperature. In addition, the transformation temperature may be further suppressed from what is found in as-cast ingots due to thermomechanical processing of the alloy. Thus, while the unique behavior of NiTi is widely exploited in applications near room temperature, the uses for a material with properties similar to Ni—Ti, but capable of being used at elevated temperatures (greater than 100° C.) would open up new applications for shape memory alloys, particularly in the aerospace, automotive, automation and controls, appliance, energy, chemical processing, heating and ventilation, safety and security, and electronics (MEMS devices) industries.
Alloying additions to NiTi are also well known and are primarily used to affect changes in the transformation temperatures of the resulting material: either to increase or decrease the transformation temperatures or change the width of the hysteresis. For example, Fe or Co substituted for Ni; or Al, Mn, V, or Cr substituted for Ti will severely depress the transformation temperatures of the resulting ternary alloys (K. H. Eckelmeyer, Scripta Metall. 10 (1976) 667 and C. M. Hwang and C. M. Wayman, Scripta Metall. 17 (1983) 1345). Cu has relatively little effect on the transformation temperatures while it can significantly reduce the hysteresis (T. Honma, M. Matsumoto, Y. Shugo, I. Yamazaki, in ICOMAT-79: Proceedings of the International Conference on Martensitic Transformations, Cambridge, Mass., (1979) pp. 259-264.) and Nb broadens the hysteresis (J. H. Yang and J. W. Simpson, J. Phys. IV 5 (1995) C8-771).
There is also a group of alloying additions (Pd, Pt, Au, Hf, and Zr) that can raise the transformation temperatures of NiTi. Current proposed alloying schemes are very specifically defined and involve either substitution of a precious metal for Ni, e.g., (Ni,Pd)Ti, (Ni,Pt)Ti, (Ni,Au)Ti (as described by Lindquist, in “Structure and Transformation Behavior of Martensitic Ti—(Ni,Pd) and Ti—(Ni,Pt) Alloys,” Thesis, University of Illinois, 1978 and Wu, “Interstitial Ordering and Martensitic Transformation of Titanium-Nickel-Gold Alloys,” Thesis, University of Illinois 1986), or substitution of a reactive element like Hf or Zr for Ti, e.g., Ni(Ti,Hf) and Ni(Ti,Zr), (as claimed by AbuJudom, II et al., U.S. Pat. No. 5,114,504).
However, having a high transformation temperature does not guarantee that the material will have specific work output acceptable for actuator use as demonstrated for Ni-30Pt-50Ti (at. %) alloys (R. Noebe, et al., SPIE Conf. Proc. Vol. 5761, (2005), pp. 364-375). Furthermore, of these high-temperature ternary NiTi-based systems, useful work characteristics have only been verified in Ni(Ti,Hf) thin films by Rasmussen et al. (U.S. Pat. No. 6,454,913 and patent Pub. No. US2002/0189719A1). To our knowledge, work measurements have not been made public on any of the other potential high-temperature NiTi-based systems. Furthermore, the Ni(Ti,Hf) alloys mentioned previously can only be used in practice in applications up to about 100° C. because of the wide hysteresis in these alloys such that the Mf is near or below 100° C., and there is little chance of increasing the use temperature (transformation temperature) of these alloys by further increasing the Hf level due to microstructural limitations. The wide hysteresis for the Ni(Ti,Hf) alloys also makes them unsuitable for applications where active control is involved.
Methods for processing HTSMA have also been described in the literature. Tuominen, et al. (U.S. Pat. No. 4,865,663 (1989)) claimed a series of (Ni,Pd)Ti—B alloys for increased fabricability. Goldberg et al. (U.S. Pat. No. 5,641,364 (1997)) have claimed a method for manufacturing and heat treating (Ni,Pd)Ti, Ni(Ti,Zr) and Ni(Ti,Hf) alloys for improved shape recovery characteristics. Rasmussen et al. (U.S. Pat. No. 6,454,913 (2002)) have claimed a method for forming thin film deposits of Ni(Ti,Hf) alloys particularly by magnetron sputtering. Johnson et al. (U.S. Pat. No. 6,669,795 (2003)) have described a method for making HTSMA thin films using multiple sputtering targets where the goal was to create an alloy that has a 1:1 atomic ratio of (Ni+X):Ti or Ni:(Ti+Y) where X is an element from the right side of the periodic table such as Pd, Pt, Au, or Cu and Y is an element from the left side of the periodic table such as Hf or Zr. Patents have also been applied for the method of producing low hysteresis (Ni,Cu)(Ti,Hf) thin films (Rasmussen et al., US patent Pub. No. 2002/0189719) and (Ni,Pd)Ti thin films (Rasmussen et al., US patent Pub. No. 2003/0168334). In all cases, compositions have been restricted to alloys containing Ti or Ti-equivalents of 50% or greater.
In all cases where high transformation temperatures are required, standard practice in the art is to maintain a ratio of (Ni+X) to (Ti+Y) (where X=Pd, Pt, Au, Cu and Y=Hf, Zr) that is stoichiometric, e.g., 1:1 or that is slightly (Ti+Y)-rich in composition. It is well known that in binary NiTi (Melton, in Engineering Aspects of Shape Memory Alloys, 1990) and in (Ni,Pd)Ti (S, Shimizu, et al., Maters. Lett, 34(1998)23) and in Ni(Ti,Hf) alloys (AbuJudom, II et al., U.S. Pat. No. 5,114,504 (1992)), that even the slightest deviation toward (Ni+X)-rich stoichiometries results in an extremely steep decrease in the transformation temperatures of the alloy negating the ability to use these alloys at high temperatures. Another consequence, of this “control” of stoichiometry is that Ti-rich alloys will contain Ti2Ni type precipitates, which form during melting as coarse globular interdendritic structures that are not amenable to control through heat treatment and at best do not benefit properties and at worst limit fatigue, fracture strength, and martensite volume fraction.